Near surface photon energy spectra outside a 6 MV field edge.

The purpose of this study was to investigate the difference between a 6 MV linear accelerator x-ray energy spectrum outside the field edge near a phantom surface, and the corresponding spectrum on the central axis. The Monte Carlo code MCNP-4A was used to calculate the spectra on the central axis and at 1, 2, 5 and 10 cm from the edge of a 4 x 4 cm2, 10 x 10 cm2 and 15 x 15 cm2 field. Compared to the spectrum on the central axis, the spectra outside the field edge showed two distinct regions: a broad peak below about 0.5 MeV, and a lower amplitude, less rapidly changing region at higher energies from 0.5 to 6 MeV. The lower energy peak was due to scattered photons, and the higher energy component was due mainly to primary photons transmitted through the jaws of the secondary collimator. The potential impact of these spectral differences on critical organ photon dosimetry was determined by calculating the ratio of the sensitivity of a Scanditronix EDD-5 diode and of a LiF:Mg:Ti thermoluminescent dosimeter (TLD) outside the field edge to their respective sensitivity at the calibration position on the central axis. The lower energy peak combined with the non-uniform energy sensitivity of each detector produced up to a two-thirds overestimate of x-ray dose outside the field by the diode, whereas the response ratio of the TLD was about unity. These results indicated that a similar evaluation was required for profile measurements of a dynamic wedged field and measurements in an intensity modulated beam with either type of detector.